1. Field of the Invention
This invention relates generally to breathing ventilators, and more particularly relates to a system and method for providing pilot pressure to a pneumatically controlled exhalation valve utilized for controlling patient end expiratory pressure (PEEP) during the expiratory cycle of a breath, and for closing the exhalation valve during inspiration, in a ventilator system having a limited air supply and having no external compressed air source for operation of the pneumatically controlled exhalation valve.
2. Description of Related Art
Medical ventilators are generally designed to ventilate a patient's lungs with breathing gas to assist a patient in breathing when the patient is somehow unable to adequately breath without assistance. Pressure assistance can be instituted when the patient has already begun an inspiratory effort, typically by bellows or fixed volume piston type ventilators. With such a system, it is desirable to immediately increase the pressure after a breath is initiated in order to reach a target pressure. This rise in pressure causes flow to be initiated in the patient airway which supplies breathing gas to the patient's lungs. Conventional pressure controlled ventilator systems terminate or reduce the flow when the target pressure is reached to limit the patient airway pressure. Patient airway pressure can be finely controlled by an exhalation valve that is typically operated by a pilot pressure supplied pressurized breathing gas from the ventilator or an external supply of pressurized or compressed air. For purposes of this description, the terms "compressed" and "pressurized" air are intended to mean any air or gas that has a pressure greater than atmospheric pressure that can be utilized as a source of pilot pressure for operation of the pilot pressure driven exhalation valve.
However, some ventilators, such as piston type ventilators for example, are generally designed to provide pressurized breathing gas on demand, and therefore are not generally suited to provide a constant supply of pilot pressure for operation of a pneumatically controlled exhalation valve. Provision of an additional supply of compressed air for operation of the pneumatically controlled exhalation valve, such as by a tank of compressed air, or an external compressor, can add significantly to the complexity, size and weight of a ventilator system, negating advantages of compactness and efficiency of a piston type ventilator. For piston type ventilator systems that are operated in emergency vehicles, or that are portable, such an external source of compressed air may not be readily available. Conventional piston type ventilators therefore commonly do not provide sufficient control of the patient airway baseline pressure, commonly known as PEEP (patient end expiratory pressure), suitable for use of such ventilators for intensive care patients. patients, it is often desirable to provide the capability of controlling and changing the PEEP during different phases of patient breathing, such as for APRV (airway pressure relief ventilation) in which two baseline pressure levels are cycled at a fixed rate in time with a constant flow rate, and a programmed cycle of patient airway pressure is superimposed over the preset baseline pressures. It would therefore be desirable to provide a method and apparatus for supplying an internal source of controllable pilot pressure for operating a pneumatically controlled exhalation valve of a piston type ventilator, allowing for PEEP to be controlled to provide APRV.